STATEMENTS OF CHANGES IN COMMON EQUITY AND COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified		Total	Common Stock [Member]	Other Paid-In Capital and Premium on Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2011		$ 464,186	$ 5,848	$ 200,926	$ (12,129)	$ 269,541
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		58,241				58,241
Other Comprehensive (Loss) Gain, Net of Tax	[1]	(1,156)			(1,156)
Cash Dividends Declared – Common Stock		0				0
Additional Investment by Shareholder		0
Excess Tax Benefit (Tax Deficiency) from Restricted Stock Plan		124		124
Additional Statement Information [Abstract]
Combined statutory rate		41.00%
Balance at Dec. 31, 2012		521,395	5,848	201,050	(13,285)	327,782
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		62,236				62,236
Other Comprehensive (Loss) Gain, Net of Tax	[1]	2,416			2,416
Cash Dividends Declared – Common Stock		0				0
Additional Investment by Shareholder		25,000		25,000
Excess Tax Benefit (Tax Deficiency) from Restricted Stock Plan		(78)		(78)
Additional Statement Information [Abstract]
Combined statutory rate		41.00%
Balance at Dec. 31, 2013		610,969	5,848	225,972	(10,869)	390,018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		66,483				66,483
Other Comprehensive (Loss) Gain, Net of Tax	[1]	(3,610)			(3,610)
Cash Dividends Declared – Common Stock		(18,201)				(18,201)
Additional Investment by Shareholder		25,000		25,000
Excess Tax Benefit (Tax Deficiency) from Restricted Stock Plan		(73)		(73)
Additional Statement Information [Abstract]
Combined statutory rate		40.00%
Balance at Dec. 31, 2014		$ 680,568	$ 5,848	$ 250,899	$ (14,479)	$ 438,300

[1] Determined using a combined statutory tax rate of 40% in 2014 and 41% in 2013 and 2012.